1. Field of the Invention
The present invention relates generally to electro-optical devices, and more specifically to modulator drivers for high-speed communications.
2. Description of Related Art
In a high-speed fiber optics communication system, the light is often transmitted into the fiber by a continuous-wave laser followed by an electro-optical modulator. The modulator, which turns the light ON and OFF to generate logical ones and zeros for transmission, is generally fabricated with a non-linear material such as Lithium Niobate (LiNbO3) or Indium Phosphide (InP). These materials exhibit a change in optical refractive index or absorption as a function of an applied voltage. Due to the weakness of such electro-optic effects, a substantial voltage is required to maximize the ratio of optical power between on and off states (the extinction ratio) of the modulated light. Moreover, since the modulator operates at or near the full bit-rate of the communications system, the electrical input to the modulator has both the largest magnitude and highest bandwidth of any signal in the system. Producing this signal requires an amplifier, known as a modulator driver, which achieves high output amplitude and high speed without degrading the quality of the transmitted pulse.
One popular type of electro-optic modulator is the Mach-Zehnder (MZ) modulator, which uses the linear electro-optic (Pockels) effectxe2x80x94commonly in LiNbO3xe2x80x94and an interferometer to generate light pulses. A typical MZ modulator requires a 6-volt peak-to-peak drive applied to a single input (xe2x80x9csingle drivexe2x80x9d) or applied differentially to, two inputs 180 degrees out of phase (xe2x80x9cdual drivexe2x80x9d). In a practical application, the modulator driver should be capable of producing 7 to 8 volts peak-to-peak to obtain adequate margin for variations due to temperature, losses in the materials, drift, etc. Electro-absorption modulators typically require single-ended drive voltages on the order of 3 to 4 volts peak-to-peak.
The challenge in modulator driver design is to create an amplifier with high voltage swing, wide bandwidth, and high-fidelity pulse response. For example, in a 40-Gigabit per second (40 Gbps) system the transmitted data signal may contain substantial energy at frequencies up to and beyond 40 GHz, depending on the rise and fall times of the data pulse edges. The modulator driver requires an amplitude response that is nearly constant versus frequency, along with constant group delay (i.e., linear phase response)xe2x80x94in other words, all frequencies are amplified equally and travel at the same speed through the amplifier. Deviations from this ideal affect the transmitted data signal, producing pattern-dependent jitter in the timing of the transitions and decreased contrast between logic states (xe2x80x9ceye closurexe2x80x9d).
One objective in driver-amplifier design is to produce a large amount of current, which typically requires large transistors. The parasitics of the transistorsxe2x80x94capacitance, resistance, and in some cases inductance, which increase with the size of the transistorxe2x80x94slow down the response, as time is wasted in transferring energy to them rather than to the actual load, the modulator. One classic solution that achieves high speed with a large effective transistor size is the distributed amplifier 100 as shown in FIG. 1, either with a single output or a differential output. This approach defines a certain transistor size that is necessary for the desired output amplitude, and then divides up a large transistor into multiple smaller transistors, which are connected by inductors in such a way that the parasitic capacitance of each small transistor in conjunction with the inductors forms a transmission line. The inductors (or high-impedance transmission line sections) allow the smaller capacitors to charge in sequence, emulating the propagation of a wave in a transmission line, thus obtaining larger bandwidth at the acceptable expense of increased delay. Thus this circuit is also known as a xe2x80x9ctraveling wave amplifier.xe2x80x9d
In recent modulator driver designs for next-generation optical networks that require transmission rates of 40G (OC-768), 50G, and beyond, the distributed amplifier approach has been widely used to achieve the required bandwidth and output amplitude. However, traveling-wave amplifiers occupy a significant amount of die space as the circuits themselves are physically large and make very inefficient use of substrate area. Furthermore, the individual frequency responses of the multiple small transistors produce numerous poles in the amplifier""s transfer function that coincide near the upper bandwidth limit of the amplifier. Beyond this cutoff frequency, the gain decreases sharply and the phase becomes highly nonlinear, creating large variations in group delay as the band edge is approached. Discontinuities and mismatches between devices in the circuit cause unwanted reflections and reactive energy storage in the transmission lines, further degrading the amplitude and phase characteristics as frequency increases. Consequently, in order to mitigate these problems and increase the usable frequency range, designers end up creating a much larger and broader-band amplifier than is necessary, thereby producing an inefficient amplifier structure.
Another shortcoming in a conventional design is that it typically requires an external means of biasing, such as a bias tee, which separates DC and AC allowing the power supply and the output signal to coincide on the same line. A bias tee is generally impossible to integrate onto a chip due to the large inductor required to meet the low-frequency requirements of optical data signals (e.g. below 50 kHz). It must be added externally, increasing assembly cost and complicating the packaging of the chip as well as degrading the response of the amplifier. Furthermore, traveling-wave amplifiers produce relatively low gain, typically requiring a pre-driver in order to operate with standard inputs such as 400-millivolt peak-to-peak current-mode logic (CML) signals from multiplexer devices and the like. Increasing the gain is difficult without sacrificing bandwidth, and often means adding another traveling-wave amplifier in series, thus multiplying the shortcomings of the architecture two-fold.
Accordingly, there is a need to design an amplifier with a large output voltage and current swing capability, having optimum amplitude and phase characteristics over a broad frequency range for driving modulators in a high-speed fiber optic system.
The present invention provides a modulator driver design that employs a differential amplifier coupled to feedback amplifiers through tuning networks. Each tuning network comprises a set of inductors that enables a broadband response, while reducing the loading effect of the feedback amplifier. An active load is placed at the output to serve multiple purposes: generating a high output voltage swing, reducing the required power supply voltage, and allowing the bias circuits to be integrated on a chip.
A modulator driver comprises: a first amplifier stage (A1) having inputs and outputs; a second amplifier stage (A2), having inputs and outputs, the inputs of the second amplifier coupled to the outputs of the first amplifier; an active load having inputs and outputs, the inputs of the active load coupled to the outputs of the amplifier stage; and a feedback stage (A3) having inputs and outputs, the inputs of the feedback stage coupled to the outputs of the second amplifier stage by means of a tuning network, and the outputs of the feedback stage coupled to the inputs of the second amplifier stage.
Advantageously, the present invention generates fully differential outputs, which enables operation with dual-drive optical modulators requiring only half the voltage swing, per input, of single-input modulators. The dual-drive capability also improves the performance of the optical system by allowing zero or non-zero adjustable chirp (phase shift) to be added to the optical signal for improved propagation in dispersive fiber.